This invention relates to liquid crystal compositions, and more particularly, to dichroic liquid crystal compositions containing perylene-based dyes.
Liquid crystals are conventionally used in conjunction with one or more dyes to obtain improved colors when liquid crystals are used in liquid crystal displays. In such cases, a host liquid crystal composition comprises a host liquid crystal material and guest dye dissolved in the host liquid crystal material. The guest dyes, which may be used alone or in combination with other dyes to achieve a desired color, preferably have certain properties which enhance their use in liquid crystal compositions. These properties include dichroism, solubility of the dye in the liquid crystal and high order parameter.
Dichroism is the property whereby an oriented assembly of dye molecules exhibit relatively low absorption of a given wavelength in one state of orientation and a relatively high absorption of the same wavelength in another state of orientation with respect to the light source. The orientation can be brought about by dissolution of the dye in a liquid crystal solvent.
Solubility must be sufficiently high so that thin layers, for example, of ten micrometers, have adequate light absorption in one of the oriented states. In certain instances, the solubility of a particular dye may be relatively low, however, the dye may still be used in conjunction with other dyes which are also soluble in the liquid crystal host so that there is adequate light absorption in one of the oriented states.
Order parameter is a quantative measure of the degree of molecular order or alignment in a given system. High order parameter is promoted by dyes with elongated shape having a large ratio of molecular length to breadth, similar to the shape of the molecules of a liquid crystal host material. To insure an elongated shape, the molecules should have a rigid structure. The brightness and contrast are both related to the order parameter, normally designated as S, of the dye, where S=(R-1)/(R+2), and R is the ratio of light absorption in the dye measured with a polarizer respectively parallel and perpendicular to the nematic director of the liquid crystal host at the wavelength of maximum absorption. Advantageously, the order parameter should be at least 0.60, and preferably, as high as possible, to achieve a minimum desired contrast ratio while still allowing a liquid crystal guest-host display to be fabricated with a reasonable brightness parameter.
The eye is sensitive to radiation in the 400-700 nm wavelength range. To fabricate a dichroic liquid crystal display with a neutral black color, for example, the radiation present in a source of illumination, such as, daylight, incandesent lamps or fluorescent lamps, to which the eye is sensitive, must be absorbed to a substantial degree at all wavelengths from 400-700 nm. Since no single dye has efficient absorption over the entire visible spectrum, mixtures of dyes of different colors are required.
Shanks, in U.S. Pat. No. 4,211,473, provides a liquid crystal display device having enhanced contrast, pleochroic and fluorescent materials being incorporated into one or more components of the device. In an example, a white card reflector located behind the liquid crystal is coated with a perylene dye which is Perilene (Fluorol Green-Gold) Color Index No. 59075. This dye absorbs light of blue coloration transmitted by the pleochroic dye and fluoresces by converting the absorbed light into light of longer wavelength (i.e., greenish yellow).
The object of this invention is to provide dichroic liquid crystal formulations utilizing a new class of red dichroic dyes which can be formulated to give useful black mixtures. This and other objects of the invention will be apparent to those skilled in the art from the following detailed description.